The invention relates to a method and an apparatus for material working or processing with the aid of a laser, and more particularly to such method and apparatus in which the laser light is directed via a laser optical system onto the material and the light reemitted or scattered back by the material is conducted via the laser optical system to a detector arrangement which is followed by an evaluating circuit for controlling the laser.
In a known method of this type (Optical Studies of Pulsed Laser Fragmentation of Biliary Calculi, Applied Physics B, Springer Publications 1987 pages 73-78) the output signal of a laser is directed via a laser optical system and an optical wave guide onto urinary and biliary calculi to fragment the latter. In the laser optical system a semireflecting mirror is disposed which conducts a part of the light reemitted, reflected back or scattered back by the stone, which is returned via the optical waveguide, onto a detector 1 which is followed by an evaluating circuit in the form of a spectral analyzer.
It is very difficult here to lead the optical waveguide in such a manner that a detrimental effect on the tissue surrounding the stone is completely excluded. A visual control or a control with the aid of radiological methods also cannot exclude with certainty that the laser energy strikes tissue parts.
The invention is based on the problem of providing a method and an apparatus of the type mentioned at the beginning in which with low expenditure limitation of the working or processing to the desired regions or materials is automatically achieved so that for example in the case of fragmentation of human stones, such as urinary and biliary calculi, damage to the tissue embedding the stone is avoided with certainty.
This problem is solved by having the amplitude-versus-time profile of the light incident on the detector arrangement evaluated after passage of a predetermined period of time from the start of the laser light, and used for controlling the laser and/or an optical switch arranged in the output beam path of the laser characterizing clause of claim 1 or 11. Advantageous embodiments and further developments of the invention are set forth in the respective subsidiary claims.
With the configuration of the method and apparatus according to the invention exactly controllable working of the material to be processed is achieved, as is essential in particular when using the method and apparatus for fragmenting human stones, for example urinary and biliary calculi. The laser may be controlled in such a manner that pulse emission which preserves the tissue, i.e. is stone-selective, is obtained. The regulation of the energy applied may be effected via changing the power density or changing the pulse duration or controlling both these parameters.
According to a further embodiment the laser pulse is interrupted before reaching its maximum energy when the output signal of the detector arrangement within a predetermined time after triggering the laser pulse does not exceed a predetermined threshold value. In this embodiment the detection according to the invention takes account of the fact that the output signal of the detector arrangement, when the laser pulse strikes hard material, assumes considerable values only a short time after triggering of the laser pulse whilst a rise of said output signal does not take place until after a considerably longer time when the pulse meets tissue material or soft material. If therefore within a predetermined time after triggering of the laser pulse a rise of the output signal has not yet taken place this means that the laser pulse is striking relatively soft material and can then be terminated before reaching its maximum value for example by closing an optical switch.
The method and apparatus according to the invention are also suitable for working other materials, for example narrowly defined regions of semiconductor materials in the fabrication of integrated circuits.
A further use is for example in angioplasty.
In the method and apparatus according to the invention the laser and the optical switch following said laser are controlled in accordance with one embodiment so that firstly only measuring pulses are emitted, their pulse energy being kept so low that no dielectric breakdown yet occurs. The time function profile of the light incident on the detector arrangement is evaluated. If this evaluation shows the presence of material to be processed or worked the laser energy is increased to ensure that the dielectric breakdown occurs only at the material to be worked, for example the stone.
Subsequently, it is then possible with each working pulse to make an analysis of the amplitude-versus-time profile of the light incident on the detector, said time function course or profile changing clearly when the laser pulse strikes surrounding material instead of the material to be worked. When this is detected the laser energy is reduced to a lower value.
It is alternatively possible to cause a measuring pulse to follow each working pulse of high energy, said measuring pulse determining whether material still to be processed is struck by the laser pulse. In each case damage to the material surrounding the material to be worked can be avoided with certainty.
Furthermore, according to a preferred embodiment it is possible with the aid of an optical switch to interrupt a laser pulse before the maximum energy is reached when the time profile of the output signal of the detector arrangement indicates that the laser pulse is not directed onto material to be worked so that damage to material which is not to be worked is avoided.
In addition to the optical evaluation the acoustic signal accompanying the dielectric breakdown can be detected and evaluated for example to determine the number of working impulses applied for documentation purposes.